1. Technical Field
The present invention relates to an image pickup apparatus. More particularly, the present invention relates to an image pickup apparatus which is applied to a video camera using a complementary mosaic color coding CCD.
2. Prior Art
A single-board CCD color camera system or the like captures color images by using a single solid-state image pickup element such as a CCD image sensor. Such a system needs to provide each pixel with a different color filter namely a color coding filter on the CCD image sensor as an image pickup element.
FIG. 1 shows an example of a color array structure used for the complementary mosaic color coding filter as an example of a color coding filter. In this figure, Cy stands for cyan, Ye for yellow, G for green, and Mg for magenta. There are lines N0 to N5. The ith line is indicated as Ni (0≦i). Each pixel is expressed as the jth pixel (0≦j). According to this convention, Xij means color X for the jth pixel on line Ni. For example, G12 means color G (green) for the second pixel on line N1. An example in FIG. 3 shows that colors are repeated horizontally at a 2-pixel interval such as Cy, Ye, Cy, Ye, and so on. Colors are repeated vertically at a 4-pixel (4-line) interval such as Cy, G, Cy, Mg, and so on. Namely, this example uses a 2-pixel interval for horizontal repetition and a 4-line interval for vertical repetition.
FIG. 2 is a block diagram of a camera signal processing system for a video camera apparatus as an image pickup apparatus. This system is provided with a CCD image sensor 102 which uses such a complementary mosaic color coding filter as illustrated in FIG. 1 or 3.
In FIG. 2, an image pickup signal from an optical system 101 is sensed in a CCD image sensor 102 and is sent to a delay circuit 120 via a front end circuit 103. The optical system 101 comprises a camera lens, a mechanical shutter, and the like. The CCD image sensor 102 functions as an image pickup element. The front end circuit 103 comprises a CDS (correlation double sampling) circuit, a GCA (gain control amplifier), an A/D (analog/digital) converter, and the like. The CCD image sensor 102 is supplied with a pulse signal read from a timing generator 106. The front end circuit 103 is supplied with a sampling pulse, an A/D converter drive pulse, and the like from the timing generator 106. The timing generator 106 is supplied with a control signal from a system control circuit 107. The delay circuit 120 is also supplied with a control signal from the system control circuit 107. An output signal from the delay circuit 120 is sent to a Y (brightness) process circuit 140 and a C (chromaticness) process circuit 160 via a preprocessing circuit 130. The Y process circuit 140 and the C process circuit 160 are also provided with a control signal from the system control circuit 107.
The system in FIG. 2 reads fields from the interlaced scanning CCD. As shown in FIG. 3, the system adds vertically adjacent pixel data to each other on every two lines in the CCD. The system reads this data in the form of a signal as illustrated in FIG. 4. This operation is called 2-line mixed reading. When the system reads every two lines N0+N1, N2+N3, and so on for one field by mixing and adding, the system then reads every two lines N1+N2, N3+N4, and so on for the next field by mixing and adding. For each pixel to be mixed and added, Cy+G is expressed as S1r, Ye+Mg as S2r, Cy+Mg as S1b, and Ye+G as S2b. 
FIG. 5 shows chromaticness signal processing including the C process circuit 160 in FIG. 2 and peripheral circuits. In FIG. 5, the delay circuit 120 retrieves a no-delay signal [HH0D], a 1H delay signal [HH1D], and a 2H delay signal [HH2D] using two 1H (one horizontal interval or one line) delay devices 121 and 122. The delay circuit then sends these signals to the preprocessing circuit 130. In the preprocessing circuit 130, an adder 131 adds the above-mentioned signals [HH0D] to [HH2D]. A ½ multiplier 132 halves these signals like ([HH0D]+[HH2D])/2 and sends the processed signal to the C process circuit 160. The delay circuit sends the 1H delay signal [HH1D] unmodified to the C process circuit 160. This signal is used for processing. In the C process circuit 160, the 1H delay signal [HH1D] and the signal ([HH0D]+[HH2D])/2 are sent to the postprocessing circuit 162 via a horizontal spatial phase synchronization filter 161. An output is retrieved from the postprocessing circuit 162. The horizontal spatial phase synchronization filter 161 uses the tap factor (1,0,3)/(3,0,1) for a filter operation. With respect to a signal for lines N2+N3 in FIG. 4, a filter operation using tap factors (1,0,3) signifies multiplying Cy20+Mg30 by the weight 1 and multiplying Cy22+Mg32 by the weight 3. (Additionally, there may be normalization through the use of division by a sum of factors.) Such a filter operation synchronizes horizontal spatial phases.
During the chromaticness signal processing in FIG. 5, delay lines of the delay circuit 120 synchronize vertical spatial phases for the signal [HH1D] and the signal ([HH0D]+[HH2D])/2. The horizontal spatial phase synchronization filter 161 synchronizes a horizontal spatial phase for each of these signals as S1 and S2. The postprocessmg circuit 162 generates a chromaticness signal by processing signals S1r, S2r, S1b, and S2b whose vertical and horizontal spatial phases are synchronized.
The above-mentioned signals [HH1D] and ([HH0D]+[HH2D])/2 are expressed as follows with regard to pixel rates for the CCD image sensor 102.[HH1D]=N2+N3([HH0D]+[HH2D])/2=((N0+N1)+(N4+N5))/2
A chromaticness signal is created at the CCD pixel rate based on data for six lines N0 to N5 in a vertical direction. Namely, the following filter operation is performed in a vertical direction according to filter tap factors.
(0,0,2,2,0,0)/(1,1,0,0,1,1)
A camera signal processing system according to the conventional field reading causes a problem during chromaticness processing. Specifically, given that “fsv” signifies a sampling frequency for the CCD pixel rate in a vertical direction, the above-mentioned filtering in the vertical direction causes a problem of generating a false color at (¼) fsv in the spatial frequency band.